1) Field of the Invention
The present invention relates to a misalignment detector for detecting misalignment of sources of laser beams that scan a photosensitive drum in an image forming apparatus.
2) Description of the Related Art
Image forming apparatuses that form one image by forming a latent image on a photosensitive drum by each of a plurality of laser beams have been disclosed in, for example, Japanese Patent Application Laid-open Publication No. 2000-267027, Japanese Patent Application Laid-open Publication No. H6-18796, and Japanese Patent Application Laid-open Publication No. H6-1002.
According to the technology disclosed in Japanese Patent Application Laid-open Publication No. 2000-267027, a plurality of writing optical systems are provided along a main scanning direction and the images are joined to thereby form a wide image. This technology makes it possible to realize a low cost.
According to the technology disclosed in Japanese Patent Application Laid-open Publication No. H6-18796, separate images are written on each of a plurality of photosensitive drums by independent laser beams, each image is developed by toners of corresponding color, and the single color images are transferred, in a superimposed manner, onto a paper to obtain a multi color image. This technology is widely known as a tandem-type image forming.
According to the technology disclosed in Japanese Patent Application Laid-open Publication No. H6-1002, a plurality of writing optical systems that irradiate different laser beams write corresponding images on one photosensitive drum.
Thus, all the technologies mentioned above use a plurality of laser beams that perform scanning. However, scanning positions of the laser beams change with the temperature or the environment. Moreover, wavelength of the laser beam also changes with a change in the temperature of a laser diode that emits the laser beam. Following problems arise if the scanning positions of the laser beam change. In the technology disclosed in the Japanese Patent Application Laid-open Publication No. 2000-267027, white lines and black lines are formed at a joint between the images resulting in a deterioration of the image. In the technologies disclosed in Japanese Patent Application Laid-open Publication No. H6-18796 and H6-1002, there is a possibility of shift in image of different colors, color unevenness, spreading of color etc. resulting in deterioration of the image.
One approach to solve these problems, as disclosed in Japanese Patent No. 3253227, is to detect a position of a mark that is formed in the image and correct the beam position according to the position of the mark. Particularly, to achieve high accuracy of detection of position-shift (particularly, to minimize an error in upward and downward movement of a pattern), a plurality of light emitting diodes (LED), each of which forms a mark (“register mark”) on the transfer belt, have been provided.
However, the requirement of a plurality of LED increases the cost. One approach to reduce the cost may be to use one LED and condense the light if that LED using a condenser lens. However, not much cost reduction is realized even with this approach. On the contrary, minimum focal length of a lens in the detecting optical system is 8 mm, shortest conjugate length (while forming a magnified image) is approximately 8×4+ distance between principal points of lens is not less than 35 mm, and including the size of a CCD and the thickness of a circuit board, the height becomes about 40 mm. As a result, the equipment becomes bulky.